Disinfection efficacy of sodium hypochlorite and glutaraldehyde and their effects on the dimensional stability and surface properties of dental impressions: a systematic review

Objective To systematically evaluate the disinfection efficacy of the two most frequently used disinfectants, sodium hypochlorite and glutaraldehyde, and their effects on the surface properties of four different dental impression materials. Methods A systematic literature search was performed in four databases until May 1st, 2022 to select the studies which evaluated disinfection efficacy of disinfectants or surface properties of dental impressions after chemical disinfection. Main results A total of 50 studies were included through electronic database searches. Of these studies, 13 studies evaluated disinfection efficacy of two disinfectants, and 39 studies evaluated their effects on the surface properties of dental impressions. A 10-minute disinfection with 0.5–1% sodium hypochlorite or 2% glutaraldehyde was effective to inactivate oral flora and common oral pathogenic bacteria. With regard to surface properties, chemical disinfection within 30 min could not alter the dimensional stability, detail reproduction and wettability of alginate and polyether impressions. However, the wettability of addition silicone impressions and the dimensional stability of condensation silicone impressions were adversely affected after chemical disinfection, while other surface properties of these two dental impressions were out of significant influence. Conclusions Alginate impressions are strongly recommended to be disinfected with 0.5% sodium hypochlorite using spray disinfection method for 10 min. Meanwhile, elastomeric impressions are strongly recommended to be disinfected with 0.5% sodium hypochlorite or 2% glutaraldehyde using immersion disinfection method for 10 min, however, polyether impression should be disinfected with 2% glutaraldehyde.


INTRODUCTION
The oral cavity is the entry portal of the gastrointestinal tract and comprised of many surfaces (Aas et al., 2005;Sampaio-Maia et al., 2016). Each of them is coated with a plethora of microorganisms, which made up the proverbial biofilm (Aas et al., 2005;Yamashita & Takeshita, 2017). Oral biofilms harbor more than 700 species of bacteria (e.g., Staphylococcus aureus, Pseudomonas aeruginosa, Bacillus atrophaeus) as well as fungi and viruses, which might cause dental and periodontal infections (Chen et al., 2010;Doddamani, Patil & Gangadhar, 2011;Maddi & Scannapieco, 2013). Previous studies have reported that oral infections were implicated in the etiopathogenesis of several important chronic systemic diseases (Chen et al., 2010;Maddi & Scannapieco, 2013).
Dental impression materials are widely used in dentistry for making accurate casts of the dentition and its neighboring oral tissues, capable of recording the prepared tooth and its surrounding anatomic topography of the desired area Khinnavar, Kumar & Nandeeshwar, 2015;Martins et al., 2017;Nimonkar et al., 2019;Piva et al., 2019;Saleh Saber, Abolfazli & Kohsoltani, 2010). During the dental impression procedure, the dental impression materials come into contact with blood and saliva, which contain potentially pathogenic micro-organisms and some viruses . Thus, in the absence of immediate disinfection, dental impressions contaminated with saliva or blood can serve as a source of cross infection between patients and dental care providers (AlZain, 2019;Amin et al., 2009;Drennon, Johnson & Powell, 1989;Karaman, Oztekin & Tekin, 2020;Khatri et al., 2020;Lad et al., 2015;Shetty, Kamat & Shetty, 2013).
The dental impressions should possess the properties of keeping stable dimensions and reproducing tiny details of the dentition and its surrounding oral tissues, which are termed as dimensional stability and detail reproduction separately Guiraldo et al., 2012). Meanwhile, it is of vital importance for dental impressions to keep their surface moist and smooth (Karaman, Oztekin & Tekin, 2020;Lad et al., 2015). The surface properties of the dental impressions might be altered during impression disinfection procedures, deteriorating the surface properties of the corresponding casts (Amin et al., 2009;. Thus, a desired disinfectant must be an effective antimicrobial agent, and in the meanwhile has no adverse effect on the impression accuracy, impression surface properties and the corresponding casts (Taylor, Wright & Maryan, 2002).
The immediate disinfection of the dental impressions has been considered as a routine procedure in dental clinics and dental laboratories (Hamedi Rad, Ghaffari & Safavi, 2010;Hiraguchi et al., 2012;Karaman, Oztekin & Tekin, 2020;Lepe et al., 2002;Shetty, Kamat & Shetty, 2013;Silva & Salvador, 2004;Thouati et al., 1996). As physical disinfections might result in temperature rise, which could cause measurable deformations in the molds Silva & Salvador, 2004). Chemical disinfectants might be more suitable for dental impression materials because they are not only widely used, but also easily managed in dental clinics. Multifarious chemicals have been used to disinfect the dental impressions, such as sodium hypochlorite, glutaraldehyde, chlorhexidine, iodophor, peracetic acid and mixed disinfectants (Amin et al., 2009;

Study selection criteria
To be eligible, studies had to satisfy all the following inclusion criteria: (1) laboratory (in vitro) studies; (2) studies based on impression materials frequently used in dentistry, including alginate, polyether, condensation silicone or addition silicone; (3) studies investigated sodium hypochlorite and glutaraldehyde as disinfectants. The disinfection methods included spray and immersion; (4) studies performed accurate evaluation of efficacy of the two disinfectants, or investigated surface properties of the dental impressions before and after disinfection, including dimensional stability, detail reproduction and wettability; (5) studies were published in English and Chinese.

Study exclusion criteria
The studies that satisfied one of the following exclusion criteria were excluded: (1) in vivo studies; (2) studies investigated on disinfectants other than sodium hypochlorite or glutaraldehyde; (3) studies based on ambiguous disinfection procedures; (4) studies based on physical disinfection methods, including ultraviolet or microwave disinfection; (5) studies did not report their measurement results; (6) studies did not have control groups; (7) studies were not reported in English and Chinese. The systematic literature search was carried out independently by two authors (Qiu Y, Xu J). If there were any disagreements, the study would be re-evaluated by a third investigator (Xu Y).

Data extraction
Different impression materials, concentrations of different disinfectants, disinfection methods, disinfection duration, surface properties and the respective results in the articles were extracted.
Data extraction was performed independently by two reviewers (Qiu Y, Xu J) and any discrepancies were resolved by consensus.

Assessment of risk of bias
To evaluate the methodological rigor of the included studies, the Joanna Briggs Institute (JBI) Critical Appraisal Checklist tool was adopted, including nine evaluation items (The Joanna Briggs Institute, 2016). Each evaluation item was rated as ''yes '', ''no'', ''unclear'', or ''not applicable''. Each study was graded according to the ''yes'' scores. Scores of 1-3 ''yes'' were considered as the high risk of bias, scores of 4-6 ''yes'' were the moderate risk of bias and scores of 7-9 'yes' were the low risk of bias.
Included studies were assessed and scored independently by two reviewers (Qiu Y, Xu J) and any disagreements were resolved by consensus.

Study selection
In total, the electronic search identified 2,044 articles from four databases (PubMed/MED-LINE, Web of Science, Embase and Cochrane Library). After duplicates had been removed, 1,825 articles were obtained. After titles and abstracts had been analyzed, 107 articles remained. After full texts were scrutinized, 50 studies were included in the analysis. Detailed study selection was presented in Fig. 1.

Notes.
a Impression materials are encoded as follows: AS, addition silicone; VPES, vinyl polyether silicone. b Dental impressions of control groups were kept in air (A), water (W) or wet environment (WE). NR: Dental impressions of control groups were not reported to be kept in air, water or wet environment. c Effects of disinfection on the surface properties of dental impressions are indicated as follows: S, significant difference was found when compared to control group; NS, no significant difference; NR, not reported.

Notes.
a Dental impressions of control groups were kept in air (A), water (W) or wet environment (WE). NR: Dental impressions of control groups were not reported to be kept in air, water or wet environment. b Effects of disinfection on the surface properties of dental impressions are indicated as follows: S, significant difference was found when compared to control group; NS, no significant difference; NR, not reported.  , 2012;Choudhury et al., 2018;Doddamani, Patil & Gangadhar, 2011;Haralur et al., 2012;Jeyapalan et al., 2018;Mendonca et al., 2013;Rajendran et al., 2021;Rentzia et al., 2011;Schwartz et al., 1994;Taylor, Wright & Maryan, 2002;Westerholm2nd et al., 1992).
Of the eight studies evaluating condensation silicone impressions, more than half of the studies suggested that disinfection procedure could significantly affect the dimensional stability Amin et al., 2009;Sabino-Silva, Jardim & Siqueira, 2020;Saleh Saber, Abolfazli & Kohsoltani, 2010;Sinobad et al., 2014;Thouati et al., 1996). Five studies reported that the use of sodium hypochlorite caused significant dimensional changes of condensation silicone impressions Saleh Saber, Abolfazli & Kohsoltani, 2010;Sinobad et al., 2014;Thouati et al., 1996). In addition, one study reported 2% glutaraldehyde immersion similarly caused adverse effects on the impressions . Other 2 studies reported that the condensation silicone impressions were not significantly affected after disinfection Silva & Salvador, 2004).
Of the seven studies investigating alginate impressions, five of them pointed out the innocuous effect of sodium hypochlorite on dimensional stability of alginate impressions, with disinfection duration ranging from 5 to 30 min (Guiraldo et al., 2012;Hamedi Rad, Ghaffari & Safavi, 2010;Hiraguchi et al., 2012;Rentzia et al., 2011). While the other two studies reported the significant difference was detected between immersion and spray disinfection methods in two experimental groups separately (Babiker, Khalifa & Alhajj, 2018;Rueggeberg et al., 1992). In addition, one study reported that 2% glutaraldehyde immersion caused significant dimensional changes while 0.13% glutaraldehyde did not ).

Detail reproduction
Six included studies investigated the ability of the dental impressions to reproduce tiny details after disinfection Guiraldo et al., 2012;Khatri et al., 2020;Nassar & Chow, 2015;Rueggeberg et al., 1992;Shambhu & Gujjari, 2010). Three studies examined whether the tiny details were completely reproduced on the dental impressions or casts when evaluating detail reproduction Guiraldo et al., 2012;Rueggeberg et al., 1992), while the other three studies assessed and ranked the reproduced details Nassar & Chow, 2015;Shambhu & Gujjari, 2010). One study reported that the detail reproduction (a width of 50-µm line) of alginate impressions was diminished, and a width of 75-µm line could be detected after 10-minute disinfection of 0.525% sodium hypochlorite using immersion or spray disinfection method (Rueggeberg et al., 1992). Instead, the other two studies showed that no detectable significant changes in detail reproduction were caused by using sodium hypochlorite at the concentration of 0.525% or 2% (Guiraldo et al., 2012;Shambhu & Gujjari, 2010). As for elastomeric impression materials, three studies reported disinfection agents could not reduce surface detail reproducibility, regardless of sodium hypochlorite and glutaraldehyde used (Guiraldo et al., 2012;Khatri et al., 2020;Nassar & Chow, 2015).

Wettability
The surface wettability of impressions was assessed in eight studies, by measuring the magnitude of the contact angle on the surface of impression before and after disinfection (Abdelaziz, Hassan & Hodges, 2004;AlZain, 2019;Kim et al., 2008;Lad et al., 2015;Lepe, Johnson & Berg, 1995;Lepe et al., 2002;Shetty, Kamat & Shetty, 2013). Of the seven studies investigating polyether impressions, five studies reported that glutaraldehyde could not affect the wettability of the impression surfaces (Abdelaziz, Hassan & Hodges, 2004;AlZain, 2019;Lad et al., 2015;Lepe, Johnson & Berg, 1995). On the contrary, one study reported the 30-minute immersion of 2% glutaraldehyde solution significantly decreased the wettability of the impression surfaces (Lepe et al., 2002), and another study reported that both sodium hypochlorite and glutaraldehyde were possible to affect the wettability (Shetty, Kamat & Shetty, 2013). As for addition silicone, four of seven studies demonstrated glutaraldehyde could significantly reduce the wettability (Abdelaziz, Hassan & Hodges, 2004;Kim et al., 2008;Lepe et al., 2002), while one study reported that 0.5% glutaraldehyde improved the wettability of addition silicone impressions effectively (AlZain, 2019). Besides, three studies reported that the wettability of impression surfaces could not be affected by glutaraldehyde and sodium hypochlorite (AlZain, 2019;Lad et al., 2015;Lepe, Johnson & Berg, 1995). In addition, one study reported the wettability of condensation silicone impressions was not affected by 10-minute immersion disinfection of 4% sodium hypochlorite or 2% glutaraldehyde (Lad et al., 2015).

DISCUSSIONS
Alginate, polyether, addition silicone and condensation silicone are four types of dental impression materials commonly used in dental clinics. Alginates are easy to use, well tolerated by patients, and excellent for primary prosthetic and orthodontic (Cervino et al., 2018). They come in the form of a powder to be mixed with water in appropriate doses (Cervino et al., 2018;Donovan & Chee, 2004). Once mixed, the alginate turns into a soft paste, and finally forms a gel within 2-5 min through a chemical irreversible reaction (Cervino et al., 2018;Donovan & Chee, 2004). The accuracy of the alginate impressions deteriorates over time because of water evaporation, thus immediate pouring of alginate impressions provides the highest accuracy regarding teeth and tissues (Garrofé et al., 2015;Guiraldo et al., 2015). Polyether, addition silicone and condensation silicone are elastomer impression materials, and most of them are provided as base/catalyst systems (Chee & Donovan, 1992;Punj, Bompolaki & Garaicoa, 2017). When used, they are entirely mixed by using some type of auto-mix system or hand mixing (Chee & Donovan, 1992;Donovan & Chee, 2004;Punj, Bompolaki & Garaicoa, 2017). Afterwards the polymerization reaction completes within several min (Donovan & Chee, 2004;Punj, Bompolaki & Garaicoa, 2017). The reaction for polyether impression materials is via cationic polymerization by opening of the reactive ethylene imine terminal rings to unite molecules without by-product formation (Sakaguchi & Powers, 2012). For addition silicone impression materials, the polymerization involves hydrosilane-terminated molecules reacting with siloxane oligomers with vinyl end groups and a platinum catalyst (Sakaguchi & Powers, 2012). Besides, the polymerization of condensation silicone impression materials occurs through a cross link between the terminal connections of the silicone polymer and an alkylic silicate (Silva & Salvador, 2004).
To obtain biologically, functionally, esthetically acceptable dental restorations, the impression materials should record the dentition and its neighboring oral tissues and transfer to the cast accurately (Karaman, Oztekin & Tekin, 2020;Perakis, Belser & Magne, 2004;Piva et al., 2019). Since disinfection is necessary for impressions to minimize the risk of disease transmission, surface properties should not be affected during disinfection procedures (Amalan, Ginjupalli & Upadhya, 2013). This systematic review comprehensively assessed the disinfection efficacy of sodium hypochlorite and glutaraldehyde, as well as the influence of different disinfectants and disinfection procedures on the surface properties of dental impressions, including dimensional stability, detail production and wettability.

Disinfection efficacy
Sodium hypochlorite and glutaraldehyde are widely used in dental clinics and dental laboratories, owing to their high antimicrobial efficacy Guiraldo et al., 2012;Khinnavar, Kumar & Nandeeshwar, 2015;. Sodium hypochlorite is effective against a broad spectrum of micro-organisms including bacteria and their spores, viruses and fungi, as well as HIV and hepatitis B virus (Guiraldo et al., 2012;Rentzia et al., 2011). Glutaraldehyde is considered a high-level disinfectant, which could eliminate some spores, the bacillus responsible for tuberculosis, vegetative bacteria, fungi, and viruses Guiraldo et al., 2012).
In addition, few studies focused the disinfection of dental impressions against viruses, so no concrete conclusions can be drawn. As far as available studies concerned, sodium hypochlorite and glutaraldehyde are effective to eliminate common infectious viruses in oral cavity Guiraldo et al., 2012;Rentzia et al., 2011). The medical instruments which carried danger of harbouring HIV or hepatitis viruses could be sterilized via chemical methods, provided all surfaces of instruments are in full contact with the disinfectants (Adler-Storthz et al., 1983;Cairns, 2000). The HIV or hepatitis B and C viruses could be inactivated by 10-minute disinfection using 0.5% sodium hypochlorite or 2% alkaline buffered glutaraldehyde (Adler-Storthz et al., 1983;Cairns, 2000). Meanwhile, a recent study reported 0.1% sodium hypochlorite or 0.5-2% glutaraldehyde could efficiently inactivate coronaviruses (Fadaei, 2021). dimensional changes might be caused by reaction of highly reactive chlorine compound of sodium hypochlorite with sulfonic ether of polyether, which could interfere with the polymerization reaction and produce distortion (Gounder & Vikas, 2016;Thouati et al., 1996). Therefore, glutaraldehyde is strongly recommended to be used for disinfecting the polyether impressions.
Addition silicone impressions seems to be the most accurate materials in comparison with others (Amin et al., 2009;Gounder & Vikas, 2016;Khinnavar, Kumar & Nandeeshwar, 2015). Traditional addition silicone is considered to be hydrophobic Khatri et al., 2020;Nimonkar et al., 2019;Queiroz et al., 2013), which highly resists to aqueous disinfectants regardless of the exposure of disinfection duration . Noticeably,  and Hiraguchi et al. (2013) pointed out that addition silicone impressions remained dimensionally accurate even after immersion for 24 h. However, nowadays, in consideration of improving the ability to reproduce details, some addition silicone impression materials are added with surfactants, which could increase the sorption of water when dental impressions are immersed for a longer period (Gounder & Vikas, 2016). Thus, prolonged disinfection duration are not clinically recommended.
Vinyl polyether silicone (VPES) has a different composition from other elastomeric impression materials as it combines vinyl polysiloxane (VPS) and polyether (PE), (Nassar et al., 2017) so it can take advantage of the properties of both PVS and PE . This review suggested that disinfection could not affect dimensional stability of VPES, because it did not contain surfactants regardless of its intrinsic hydrophilicity Nassar & Chow, 2015;Nassar et al., 2017). However, whether longer disinfection duration could affect dimensional stability is uncertain. Further studies are needed in future (Nassar & Chow, 2015).
Condensation silicone impressions presented less accurate results in dimensional stability in comparison with other dental elastomeric impression materials (Amin et al., 2009;, which might be attributed to the inherent property of the condensation silicone rather than to the disinfection (Amin et al., 2009). Owing to its hydrophobic nature, condensation silicone impressions are less susceptible to water sorption and react with disinfectants . Besides, the polymerization reaction of elastomeric compounds results in the formation of a three-dimensional net, and ethyl alcohol is formed as a by-product (Silva & Salvador, 2004). Volume reduction due to the cross link and alcohol evaporation made condensation silicone impressions exhibit some degrees of contraction, sometimes uncontrollably, during polymerization (Amin et al., 2009;Saleh Saber, Abolfazli & Kohsoltani, 2010;Silva & Salvador, 2004;Sinobad et al., 2014). As some studies suggested, immersion disinfection inhibited the vaporization of alcohol and improved the dimensional accuracy Saleh Saber, Abolfazli & Kohsoltani, 2010;Silva & Salvador, 2004). Even though significant changes between experimental and control groups were detected, ), Saleh Saber, Abolfazli & Kohsoltani (2010 and Thouati et al. (1996) considered that these changes In general, common disinfection procedures would not adversely affect the detail reproduction of the dental impressions. The wettability of the addition silicone impressions might be decreased after disinfection, while other impression materials are unaffected.

Limitations
This review merely analyzed two most commonly used disinfectants, sodium hypochlorite and glutaraldehyde. Other chemical disinfectants, such as chlorhexidine, iodophor and peracetic acid, should be further studied in future.

CONCLUSIONS
Based on the systematic review, the following conclusions could be drawn: 0.5-1% sodium hypochlorite and 2% glutaraldehyde not only could inactivate oral flora and common oral pathogenic bacteria, but also could not alter the surface properties of dental impression materials within 30 min of disinfection duration, except for the wettability of addition silicone impressions and dimensional stability of condensation silicone impressions. Therefore, the disinfection methods for dental impression materials which strongly recommended by this study are listed in Table 8.

ADDITIONAL INFORMATION AND DECLARATIONS Funding
The authors received no funding for this work.